A great number of well bore operations are conducted by well tools which are dependently coupled to a so-called "wireline" or suspension cable that is spooled on a winch at the surface which is selectively operated for transporting one or more so-called "wireline tools" between the surface and various depth locations in a well bore. Electrical conductors are provided in the cable for carrying control and measurement signals between associated surface equipment and the wireline tools as well as transmitting electrical power to electrically-actuated devices on the tools as required for effecting their particular functions.
It will, of course, be recognized that electrically-actuated explosive devices are commonly utilized with wireline tools such as perforating guns, explosive backoff tools as well as chemical and explosive cutting tools. Typically, an electrically-actuated detonating system is selectively operated for supplying power to detonate the explosive devices on the wireline tool once it has been positioned in a well bore. These detonating systems are usually comprised of an encapsulated electrically-responsive detonator that has a sensitive primary explosive cooperatively arranged for setting off a secondary explosive which, in turn, detonates the more-powerful explosive devices on the tool. For example, a typical wireline perforator utilizes an electrically-initiated detonator for setting off an explosive device such as a booster charge or a detonating cord operatively coupled between the detonator and one or more shaped explosive charges carried by the perforator. Explosive pipe-cutting tools commonly use an electrical detonator and a detonating cord for initiating the operation of an annular shaped explosive charge to produce an omnidirectional planar cutting jet for severing a pipe string. Similarly, an explosive backoff tool employs a bundled detonating cord which is actuated by an electrically-responsive detonator. The typical wireline chemical cutter employs an electrically-responsive detonator that ingnites a gas-producing propellant composition that functions to discharge pressured jets of halogen fluoride chemicals against an adjacent well bore surface.
Those skilled in the art clearly appreciate that inadvertent actuation of the electrically-responsive explosive devices on a wireline tool while the tool is located at the surface may result in fatalities and injuries to personnel as well as serious damage to the nearby equipment. One of the most common sources for the premature actuation of a wireline tool utilizing an electric detonator is, of course, the careless connection of an electrical power source to the cable conductors after the well tool has been connected to the suspension cable and the tool is still at the surface. To at least mimimize these risks, the usual practice is to delay the installation of a detonator into the tool as well as the connection of its electrical leads to the cable conductors as long as is reasonably possible. Added protection is provided by controlling the surface power source with a key-operated switch which is not unlocked until the tool is situated at a safe depth in the well bore. In some cases, a pressure-sensitive arming switch is arranged in the downhole firing circuit of a perforator which will not be closed to disable the perforator until it has reached a selected depth.
These various safety devices and procedures will, of course, greatly reduce the chances that the explosive devices carried by these wireline tools will be inadvertently detonated while they are at the surface. Nevertheless, the electrically-initiated detonators commonly used in these well tools are susceptible to being inadvertently detonated by strong electromagnetic fields which may be picked up by the conductors in the suspension cable. For example, premature actuation of a detonator may be caused by lightning for by the unpredictable presence of so-called "stray voltage" which may sporadically appear at various locations in the structure of the drilling platform. These hazards will also be present when a wireline tool having an unfired detonator and one or more unexpended explosive devices is removed from the well bore. This latter situation itself presents and additional hazard since it is not always possible to know whether or not a wireline tool that is being returned to the surface is carrying unexpended electric detonators or explosive devices.
Because of these potential hazards that exist once a tool is armed, many proposals have been made heretofore for appropriate safeguards and precautions for handling these tools while they are at the surface. For instance, when a wireline tool with an electric detonator is being prepared for lowering into a well, in keeping with the susceptibility of electric detonators to strong electromagnetic fields it usually necessary to maintain strict radio and radar silence in the vicinity. Ordinarily a temporary restriction on nearby radio transmissions will not represent a significant problem on a lang rig. On the other hand, whenever service tools with explosives are being used on a drilling vessel or an offshore platform, it is common practice to restrict, if not prohibit, the radio and radar transmissions from the platform as well as from helicopters and surface vessels in the vicinity. Similarly, it may be advisable to postpone any electrical welding operations on the rig or platform since welding machines develop erratic currents in the structure that may inadvertently initiate a sensitive electrical detonator in an unprotected wireline tool at the surface. These delays will have obvious restrictive and costly effects on the operations on many of the other platforms and drilling vessels in the vicinity.
Those skilled in the art will appreciate, of course, that many of these hazards are avoided by employing tubing-conveyed or so-called "TCP" perforating tools that do not require sensitive electrical detonators. These perforators are comprised of an upper body or so-called "firing head" with an impact-actuated detonator and a depending lower body carrying shaped explosive charges. Typical TCP perforators are fully described in, for example, U.S. Pat. No. 4,509,604, U.S. Pat. No. 4,610,312 or U.S. Pat. No. 4,611,660. These perforators are utilized by dependently coupling the perforator to a tubing joint and the lowering the perforator to a selected depth location in a well bore as the supporting tubing string is progressively assembled. In some cases, the perforator is seated on a packer assembly which has been previously set in the well bore for isolating the well bore interval that is to be perforated. The packer assembly is arranged so that once the upper body of the perforator has landed on the packer, the lower body of the tool will be situated below the packer for positioning the shaped charges as necessary for perforating the isolated interval. In other situations, the tubing string is progressively assembled until the perforator is positioned in the selected well bore interval. In either case, once the perforator is positioned at a selected depth location in the well bore, the perforator is selectively initiated from the surface either by dropping a so-called "drop bar" through the tubing string for striking the impact-responsive detonator in the firing head or by varying the pressures inside of the tubing string and in the well bore until a predetermined pressure level or differential is attained for actuating the detonator and setting off the shaped charges in the perforator.
It must be realized, however, that although TCP perforators are relatively unaffected by many of the hazards associated with electrical detonators, measurements representative of various well bore conditions can not be conveniently monitored at the surface. Accordingly, to obtain these measurements as well as to control the firing of the TCP perforator without compromising on safety, a unique transformer coupling system arranged in keeping with the principles of U.S. Pat. No. 4,806,928 (which is hereby incorporated herein by reference) has been effectively employed for inductively coupling a typical wireline cable to a TCP perforator after the perforator has been installed in the well bore. As described in more detail in that patent, this unique coupling system facilitates the transmission of electric power as well as data and/or control signals between the surface equipment and the perforator. Nevertheless, despite the several obvious advantages of that unique coupling system, it has been found that the anchoring device disclosed in that patent may be inadequate to withstand the extreme upward forces that might be produced by exceptionally-high flow rates or the detonation of a great number of powerful explosive charges on the TCP perforator.